1. Field of the Invention:
The invention relates to the manufacture of a sheet of polyvinylbutyral intended to be used as an intermediate layer in laminated glazings.
2. Background of the Prior Art:
Sheets of polyvinylbutyral are manufactured continuously, generally using an extrusion drawplate, to form a ribbon which is rolled at the end of the manufacturing line so as to facilitate its stocking and transport to the use location for the manufacture of the laminated glazings. At this location, the ribbon is cut into rough shapes for assembly with the other constituent components of the laminated glazings.
At the time of stacking the components, the sheet of intermediate plastic material is therefore in the form of a generally trapezoidal rough shape, the size of which is slightly larger than that of the glazing. After the stacking and before the final assembly, which generally uses calendering by passage of the stack between press rollers and an autoclave cycle, the parts of the intermediate sheet which extend beyond the glazing are cut by trimming with a blade which is supported by the edge of the glazing. The glazing then undergoes its autoclave cycle.
This type of assembly requires the intermediate layer to have dimensional stability. However, in certain glazings shrinkage of this intermediate layer has been noted, particularly during the calendering, which is essentially due to the existence of stresses imparted to the layer during manufacturing, particularly traction stresses due to the method of manufacturing the polyvinylbutyral sheet, which continue to exist after stocking and unrolling the ribbon. The shrinkage of the intermediate sheet results in an absence of material along the edges of the glazing, particularly in the corners, which can permit the entry of the mastic used for glue mounting the glazing in the bay, and/or a weakening of the adhesion of the laminate components with one another. Finally, the shrinkage of the sheet can cause the glazing to be rejected.
To overcome this problem of shrinkage of the sheet during assembly with the other constituent components of the glazings, the PVB sheet has been submitted to a relaxation cycle between the unrolling of the sheet after stocking and its use as an intermediate in the manufacture of the laminated glazings. This cycle corresponds to the sheet in rough shape form remaining up to several days in an adapted location, at a normal temperature, that is, up to approximately 20.degree. C. This solution is economically unsatisfactory. In addition, it does not solve another common problem, that of the adhesion of the sheet to itself in the heart of the roll during stocking Although fixed before rolling at a temperature of approximately 10.degree. C. or less and kept stocked at that temperature, the sheet is subject to shrinkage during stocking, such that the successive layers adhere to one another, particularly in the heart of the roll. It is then necessary to exert considerable traction forces on the sheet in order to unroll it. There then occurs a stretching of the sheet., creating additional stresses, and finally even greater shrinkage when the stresses are released. In certain cases the adhesion is such that unrolling is impossible.
In European patent application Ser. No. 0 021 667, in order to reduce the shrinkage of the PVB sheet, it has already been proposed to exert a constant tension, between 35 and 173 grams per linear centimeter, on the polyvinylbutyral sheet, then to roll the sheet while maintaining this same tension, and to stock the roll at a temperature below 15.degree. C. before its use. By this method, a reduction is achieved in the shrinkage of the polyvinylbutyral sheet in the assembly during the manufacture of the laminated glazing. However, this method is not fully satisfactory, especially in the case of the manufacture of a polyvinylbutyral ribbon of a very resilient nature. Shrinkage continues to exist with the above indicated disadvantages.